The lifetime risk for developing invasive breast cancer in the Western world is about 12%, and 1 in 33 women will die of breast cancer (Pharoah et al., The Breast. 1998; 7:255-259). Epithelial ovarian cancer (EOC) is the fifth leading cause of cancer-related deaths in women in the United States and the leading cause of gynecologic cancer-related deaths (Jemal A, Siegel et al., Cancer Statistics, 2007). Most breast cancer cases are sporadic, with no family history of cancer. In up to 10% of breast cancer cases a family history can be elicited, and in a subset of these individuals, germline mutations can be detected, primarily in either the BRCA1 or BRCA2 genes.
Carriers of germline BRCA1 (MIM #113705) or BRCA2 (MIM #600185) gene mutations are clearly at an increased risk for developing breast and ovarian cancer, with an estimated lifetime risk of ˜80% for developing breast cancer and ˜50% for ovarian cancer, rates that are 6-7- and 30-fold increased, respectively, over those of the general population (Wacholder et al., Science 2004; 306:2187-2191). The risk of breast cancer increases with age. A woman in the general population faces about a 12% lifetime risk of developing breast cancer. This risk remains low before age 50; the majority of risk occurs after age 60. Women with a BRCA1 or BRCA2 mutation have a much higher lifetime risk for breast cancer, and much of the risk occurs at a younger age.
There is substantial variability in the penetrance of breast cancer in BRCA1/2 mutation carriers, even among carriers of an identical mutation within families. These different penetrance rates, combined with the variability in age at diagnosis in affected mutation carriers, may imply that modifier factors—genetic and environmental—are operative in BRCA1/2 carriers to affect penetrance. Over the years, a host of environmental and genetic factors have been evaluated as putative modifiers of BRCA1/2 mutations: environmental exposures (e.g., irradiation), personal habits (e.g., smoking), lifestyle (e.g., involvement in sports), reproductive factors (e.g., age at first menstrual period), as well as the action of additional so-called “modifier genes” (e.g., a single SNP in the RAD51 gene). Despite substantial and extensive studies, few factors have emerged as “true modifiers” by virtue of reproducibility and independent validation.
Modification of breast cancer risk in BRCA1/2 mutation carriers by other genes have been proposed and investigated. Such an evaluation involves a case-control study design that determines the rates of either known functional polymorphisms or single nucleotide polymorphisms (SNPs) within and around candidate genes and compares these rates between affected and unaffected BRCA gene mutation carriers. The putative role that aberrant gene silencing by miRNA plays in affecting mutant BRCA allele penetrance has not been studied.
MicroRNAs (miRNAs, miRs) are single-stranded RNA molecules of about 21-23 nucleotides in length thought to regulate the expression of other genes. miRNAs are encoded by genes that are transcribed from DNA but not translated into protein. miRNAs are first transcribed as pri-miRNA and are subsequently processed to short stem-loop structures, pre-miRNA, in the cell nucleus. These pre-miRNAs are then processed to mature miRNAs in the cytoplasm by interaction with the endonuclease Dicer, which also initiates the formation of the RNA-induced silencing complex (RISC).
In order to function in gene regulation, a miRNA is complementary to a part of one or more mRNAs. The miRNA with the complex RISC is guided to target sequences located at the 3′-terminal untranslated regions (3′-UTRs) of mRNAs by base pairing. Annealing of miRNA to mRNA inhibits protein translation by cleavage of the mRNA through a process similar to RNA interference (RNAi), or by blocking the protein translation machinery without causing the mRNA to be degraded (Meister and Tuschl, Nature 2004; 431:343-349). Accumulating evidence has revealed that 7 nt at the 5′-terminus of an miRNA, from position 2 to position 8, called the ‘seed’ region, are essential for their function (Brennecke et al., PLoS Biol 2005; 3:e85).
MicroRNA regulation has a major impact on the proper regulation of a cell, in particular, cellular proliferation and differentiation. There is evidence that the expression level of several genes and proteins in tumors is also partially regulated by miRNA. Let-7, targeting the oncogene RAS, is down-regulated in lung cancers (Takamizawa et al., Cancer Res 2004; 64:3753-3756). Furthermore, a germline mutation in the pri-miR-16-1/15a precursor was found to cause its reduced transcription in a patient with familial CLL (Calin et al., N Engl J Med 2005; 353:1793-1801).
A SNP located in the miRNA-binding site of a miRNA target may disrupt miRNA-target interaction, resulting in the deregulation of target gene expression. Such SNP-associated deregulation of the expression of an oncogene or tumor suppressor gene might contribute to tumorigenesis. In this hypothetical model, aberrant miRNA binding, affect gene expression patterns in a way that abrogates their ability to fulfill their designated biological role as translation regulators. Such an effect may hypothetically modify cancer risk in BRCA1 and BRCA2 mutation carriers. This putative involvement of miRNA in modifying cancer risk can be detected by analysis of SNPs in miRNA binding sites and/or miRNA precursors.
There is an unmet need for detecting increased susceptibility to breast and ovarian cancer in subjects carrying a BRCA mutation, so that more accurate risk assessment becomes possible. Such information could have significant implications in terms of genetic counseling.